I've noticed this issue as well and it seems to correlate with the increasing temperature. I have been able to fix the issue by making small adjustments with the steering line tension knob.
Dead on. That is what the tension knob is for. This is an issue with older boats which have the steering attached to the seat. That system uses tubes to direct the lines under the seat and back to the hull/ Those tubes do shrink and expand with heat. That tensions or loosens the lines. Adjusting with the knob will compensate for those changes.
You just need to make sure you setup your lines to allow for some adjustability either way. If I remember right, my steering came setup initially with the adjustment knob at neutral. This meant that either direction I turned the knob, it tightened the lines. The steering line just passes through a metal post inside the steering mechanism and then wraps around the post when you turn the knob. I had to untie the lines from the steering drum under the cover at the back of the boat and then turn the line tension adjustment knob to wrap the line around the metal post once. This gives me a full turn of the knob to loosen the tesion on the lines as well at the abilility to increase the tension. Then I went through the process of centering the rudder and the steering knob and reconnecting the lines to the steering hub. If you don't put at least a single turn of line on the adjustment post, then it will only tighten the tension either way you turn the knob. I hope this makes sense!
If you have not looked at the inside of the steering control, I have some pictures posted here http://www.hobiecat.com/forums/viewtopic.php?f=78&t=22285